Synergistically optimizing electrical and thermal transport properties of Bi2O2Se ceramics by Te‐substitution

Bi₂O₂Se oxyselenides, characterized with intrinsically low lattice thermal conductivity and large Seebeck coefficient, are potential n‐type thermoelectric material in the mediate temperature range. Given the low carrier concentration of ~10¹⁵ cm^?3 at 300 K, the intrinsically low electrical conductivity actually hinders further enhancement of their thermoelectric performance. In this work, the isovalent Te‐substitution of Se plays an effective role in narrowing the band gap, which notably increases the carrier concentration to ~10¹⁸ cm^?3 at 300 K and the electron conduction activation energy has been lowered significantly from 0.33 to 0.14 eV. As a consequence, the power factor has been improved from 104 μW·K^?2·m^?1 for pristine Bi₂O₂Se to 297 μW·K^?2·m^?1 for Bi₂O₂Se_0.96Te_0.04 at 823 K. Meanwhile, the suppressed lattice thermal conductivity derives from the introduced point defects by heavier Te atoms. The gradually decreased phonon mean free path reflects the increasingly intense phonon scattering. Ultimately, the ZT value attains 0.28 for Bi₂O₂Se_0.96Te_0.04 at 823 K, an enhancement by a factor of ~2 as compared to that of pristine Bi₂O₂Se. This study has demonstrated that Te‐substitution of Se could synergistically optimize the electrical and thermal properties thus effectively enhancing the thermoelectric performance of Bi₂O₂Se.     

Enhanced thermoelectric performance of n‐type Bi2O2Se by Cl‐doping at Se site

The n‐type polycrystalline Bi₂O₂Se_1?xCl_x (0≤x≤0.04) samples were fabricated through solid‐state reaction followed by spark plasma sintering. The carrier concentration was markedly increased to 1.38×10²⁰ cm^?3 by 1.5% Cl doping. The maximum electrical conductivity is 213.0 S/cm for x=0.015 at 823 K, which is much larger than 6.2 S/cm for pristine Bi₂O₂Se. Furthermore, the considerable enhancement of the electrical conductivity outweighs the moderate reduction of the Seebeck coefficient by Cl doping, thus contributing to a high power factor of 244.40 μ·WK^?2·m^?1 at 823 K. Coupled with the intrinsically suppressed thermal conductivity originating from the low velocity of sound and Young's modulus, a ZT of 0.23 at 823 K for Bi₂O₂Se_0.985Cl_0.015 was achieved, which is almost threefold the value attained in pristine Bi₂O₂Se. It reveals that Se‐site doping can be an effective strategy for improving the thermoelectric performance of the layered Bi₂O₂Se bulks.     

Boosting the thermoelectric performance of Bi2O2Se by isovalent doping

N‐type Bi₂O₂Se has a bright prospect for mid‐temperature thermoelectric applications on account of the intrinsically low thermal conductivity. However, the low carrier concentration of Bi₂O₂Se (~10¹⁵ cm^?3) severely limits its thermoelectric performance. Herein, the boosting of the carrier concentration to ~10¹⁹ cm^?3 can be realized in our La‐doped Bi₂O₂Se ceramic samples, which could be ascribed to the formation of isoelectronic traps and the narrowing of band gap, and contribute to a marked increase in the electrical conductivity (from 0.03 S cm^?1 to 182 S cm^?1). Our X‐ray absorption near‐edge structure spectra results reveal that a local disordering of oxygen atoms could be an important reason for the intrinsically low thermal conductivity of Bi₂O₂Se, and the point defects can also suppress the lattice thermal conductivity in La‐doped Bi₂O₂Se. The ZT value can be enhanced by a factor of ~4.5 to 0.35 at 823 K for Bi_1.98La_0.02O₂Se as compared to the pristine Bi₂O₂Se. The coordinated optimization of electrical and thermal properties demonstrates an effective method for the rational design of high‐performance thermoelectric materials.     

Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi₂Te_2.7Se_0.3) and n-type bismuth telluride (Bi₂Te₃) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi₂Te₃ matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK² and 3179 μW/mK² for pure and Se-doped Bi₂Te₃ compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi₂Te₃ was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.     

Enhanced Thermoelectric Properties of Bi2O2Se Ceramics by Bi Deficiencies

Polycrystalline Bi_2?xO₂Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773 K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change of effective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is ?568.8 μV/K for Bi_1.9O₂Se at 773 K, much larger than ?445.6 μV/K for pristine Bi₂O₂Se. Featured with very low thermal conductivity [~0.6 W·(m·K)^?1] and an optimized electrical conductivity, ZT at 773 K is significantly increased from 0.05 for pristine Bi₂O₂Se to 0.12 for Bi_1.9O₂Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.     

Enhanced thermal stability of Bi2Te3-based alloys via interface engineering with atomic layer deposition

The ease of Te sublimation from Bi₂Te₃-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi₂Te₃-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi₂Te_2.7Se_0.3 powders resulted in significant suppression of the generation of pores in Bi₂Te_2.7Se_0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi₂Te_2.7Se_0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi₂Te₃-based alloys without sacrificing thermoelectric performance.     

Thermoelectric transport and magnetoresistance of electrochemical deposited Bi2Te3 films at micrometer thickness

Bismuth telluride (Bi₂Te₃) is so far the best thermoelectric material for applications near room temperature, and also exhibits large magnetoresistance. While the electrochemical deposition approach can achieve effective growth of the Bi₂Te₃ films at micrometer thickness, the magnetoresistance transportation behavior of the electrochemically deposited Bi₂Te₃ films is yet not clear. In this work, we demonstrate the thermoelectric and magnetoresistance behaviors of the micrometer thick Bi₂Te₃ films deposited via electrochemical deposition approach. The optimum thermoelectric power factor is observed in the Bi₂Te₃ sample with electrochemical deposition thickness of ~6 μm followed by rapid photon annealing treatment, reaching the magnitude of ~1 μWcm⁻¹K⁻² that is similar to the previous reports. In contrast to the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃ films, the electronic transportations of the electrochemically deposited Bi₂Te₃ are more influenced by the carrier scatterings by the grain boundaries and lattice defect. As a result, their magnetoresistance (MR) shows a distinguished non-monotonic behavior when varying the magnetic field, while the magnitude of their MR exhibits a positive temperature dependence. These MR behaviors largely differ to the previously reported ones from the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃, in which cases their MR monotonically increases with the magnetic field and exhibits negative temperature dependence. This work reveals the previously overlooked role of grain boundary that also regulates the transportation properties of bismuth chalcogenides in the presence of magnetic field.     

A Study on Crystallization Kinetics of Thermoelectric Bi2Se3 Crystals in Ge–Se–Bi Chalcogenide Glasses by Differential Scanning Calorimeter

Novel glass‐ceramics with embedded thermoelectric Bi₂Se₃ crystals were prepared from glass matrices in the Ge₂₀Se_100?xBi_x (x = 5, 10, 12 mol%) system. Based on DSC results performed at different heating rates, characteristic activation energies (E_c) and Avrami exponents (n) were obtained and analyzed by using Kissinger's relation, Ozawa's method, Augis–Bennett approximation and Matusita–Sakka theory. XRD results showed that pure Bi₂Se₃ crystalline phase precipitated upon annealing at different temperatures for various time. The crystal size and crystalline fraction in the samples could be tuned by controlling the annealing time.     

High thermoelectric performance of (Bi1-xPrx)2(Te0.9Se0.1)3 alloys prepared by high-pressure sintering method

In this article, n-type (Bi_1-xPr_x)₂(Te_0.9Se_0.1)₃ (x = 0, .002, .004, .008) alloys were fabricated by high-pressure sintering (HPS) method together with annealing. The effect of high pressure and Pr contents on the microstructure and thermoelectric performance of samples were explored in detail. The results show that the HPS samples are composed of nanoparticles. Pr doping has significant impacts on the electrical and thermal transport properties of the Bi₂Te_2.7Se_0.3 alloys. The HPS sample with x = .004 shows the maximum ZT value of .31 at 473 K, which is enhanced by 41% to compare with the Pr-free sample. Annealing can improve the thermoelectric properties by increasing the electrical transport properties and decreasing the thermal conductivity simultaneously. As a result, the highest ZT value of 1.06 is achieved for the annealed sample with x = .004 at 373 K, which is beneficial to the thermoelectric power generation.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Effects of sulfur substitution for oxygen on the thermoelectric properties of Bi2O2Se

In the present work, the thermoelectric properties of S-doped Bi₂O_2-xS_xSe at the temperatures from 320 to 793 K have been studied. The results show that the solubility limit of S is around x = 0.01 and S-doping is helpful to the sintering and grain growth of Bi₂O₂Se. Moreover, S-doping reduces the band gap of Bi₂O_2-xS_xSe remarkably as x rises. As a result, a thousand times promotion of electrical conductivity at x = 0.02 is obtained, leading to a nearly 3 times increase of power factor at 787 K. By virtue of the intrinsically low thermal conductivity, a peak ZT of 0.29 at 793 K with an average of 0.21 has been achieved for Bi₂O_1.98S_0.02Se, which is nearly 3 and 6 times larger than that of the pristine one. This study indicates that a small amount of S substitution for O could improve the thermoelectric properties of Bi₂O₂Se effectively.     

Origin of improved average power factor and mechanical properties of SnTe with high-dose Bi2Te3 alloying

Chemical bond and defect engineering have profound impact on energy band and crystal structure of materials, which can adjust physicochemical and mechanical properties of materials. Bi_Sn and V_Sn are simultaneously designed in SnTe material system via the (SnTe)_x(Bi₂Te₃) alloying form, which was realized by vacuum melting and spark plasma sintering technology. Both point defects increase band gap and decrease ΔE_L−Σ, which improves obviously Seebeck coefficient of SnTe. The high-efficiency improvement in the middle temperature zone makes average power factor of (SnTe)₂₄(Bi₂Te₃) achieves 1656 μWm^-1K⁻²(323K–773K). Bi_Sn induces strong coupling hybridization between Te-5s and Te-5p, and forms the lower bonding state. The lower bonding state and the larger -IpCOHP increase bond strengthen, which result in the smaller thermal expansion coefficient and the higher hardness. Bonding evolution makes thermal expansion coefficient of SnTe reduce by 27%, and Vickers hardness increase by 44%. Higher average power factor, lower thermal expansion coefficient and higher hardness can improve output power density and service life, which provides strategies for exploiting high-power thermoelectric devices with suitable mechanical properties.     

Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se

The Bi₂O₂Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 10¹⁵?cm^?3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi₂O₂Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03?S/cm in pristine sample to 133?S/cm in x?=?0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823?K for Bi_1.92Ge_0.08O₂Se, which is the highest ZT value for Bi₂O₂Se-based thermoelectric materials.     

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi₂Te_2.7Se_0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi₂Te_2.7Se_0.3 system is optimized by 2D Ti₃C₂T_x additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm^?1K^?2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm^?1K^?1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti₃C₂T_x nanosheets and Bi₂Te_2.7Se_0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi₂Te_2.7Se_0.3/0.1 wt% Ti₃C₂T_x, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi₂Te_2.7Se_0.3 materials.     

Low thermal conductivity and thermoelectric properties of Si80Ge20 dispersed Bi2Sr2Co2Oy ceramics

Bi₂Sr₂Co₂O_y is a thermoelectric material with low thermal conductivity. The Bi₂Sr₂Co₂O_y/Si₈₀Ge₂₀ composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si₈₀Ge₂₀ alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si₈₀Ge₂₀ in the Bi₂Sr₂Co₂O_y matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi₂Sr₂Co₂O_y + 0.2 wt% Si₈₀Ge₂₀ sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi₂Sr₂Co₂O_y sample.     

Sodium Doping Effects on Layered Cobaltate Bi2Sr2Co2Oy Thin Films

Highly c‐axis oriented Bi_2?xNa_xSr₂Co₂O_y (x = 0, 0.1, 0.2, 0.3) thin films were successfully deposited onto LaAlO₃ (0 0 1) single crystal substrates by the chemical solution deposition method. The Na‐doping effects on microstructures as well as transport and thermoelectric properties were investigated. The crystallite size normal to the thin film surface was decreased with Na doping, and the carrier concentration and mobility was enhanced and decreased, respectively. As for the transport properties, it is suggested that Na‐doping‐induced disorders play a more important role at low temperatures, while at the medium temperature the doping play a trivial role, at higher temperatures Na‐doping weakens the electron correlation. Combined with the transport properties and Seebeck coefficient results, it is suggested that the thermoelectric properties are mainly controlled by carrier concentration due to the hole doping in blocking layers. Power factor at 300 K was enhanced about 22% for the Bi_1.7Na_0.3Sr₂Co₂O_y thin film as compared with that of the undoped thin films, suggesting that Na doping at blocking layer in Bi₂Sr₂Co₂O_y was an effective route to enhance the thermoelectric power factor.     

Synthesis of n-type Bi2Te1-xSex compounds through oxide reduction process and related thermoelectric properties

We propose a new process for the fabrication of n-type Bi₂Te_3-xSe_x (x = 0, 0.25, 0.4, 0.7) compounds. The compounds could be synthesized successfully using only oxide powders as the starting materials via the mechanical milling, oxidation, reduction, and spark plasma sintering processes. The controllability of the Se content could be ascertained by structural, electrical, and thermal characterizations, and the highest thermoelectric figure of merit (ZT) of 0.84 was achieved in Bi₂Te_2.6Se_0.4 compound at 423 K without any intentional doping. This process provides a new route to fabricate n-type Bi₂Te_3-xSe_x compounds with competitive ZTs using all oxide starting materials.     

Mo doping-enhanced dye absorption of Bi2Se3 nanoflowers

A simple solvothermal approach is explored to prepare Bi_2−xMo_xSe₃ nanostructures by employing N,N-dimethylformamide (DMF) as the solvent. Mo plays an important role in the assembly of the Bi_2−xMo_xSe₃ nanostructures from nanoplates to nanoflowers. Structural and morphological studies indicate that the resulting products are large specific surface area single-crystalline Bi_2−xMo_xSe₃ nanoflowers self-assembled from thin nanoplates during the reaction process. The absorption properties of the as-prepared samples are investigated with Rhodamine B (RhB) as dye, and it is found that the Bi_1.85Mo_0.15Se₃ nanoflowers show an optimal adsorption capacity, implying that Mo doping not only changes the morphologies of the nanostructures but also enhances their absorption behaviors.     

Transport properties for Bi2Se3/La0.7Sr0.3MnO3 composites

Topological insulator/perovskite manganese oxide composites (Bi₂Se₃/La_0.7Sr_0.3MnO₃) were prepared using the bonded-exfoliated method and the transport properties were studied. The resistivity of the bonded manganese oxide substrate was 6 orders of magnitude larger than that of a Bi₂Se₃ single crystal, implying that the bonded manganese oxide acted as an insulator substrate. The intrinsic resistance peak of the manganese oxide was present in the temperature-dependent resistivity curve. Metal-insulator transition induced by the magnetic field at a low temperature was observed. The simple parallel model cannot account for the electrical properties, as the conducting manganese oxide grains were bonded by polymeric silicone rubber and resistivity peaks were present only when the thickness of the Bi₂Se₃ layer was less than 1.043 μm. The spin injection may be present in the composite structure.     

Electrodeposition and characterization of Bi2Se3 thin films by electrochemical atomic layer epitaxy (ECALE)

Bismuth selenide thin films were grown on Pt substrate via the route of electrochemical atomic layer epitaxy (ECALE) in this work for the first time. The electrochemical behaviors of Bi and Se on bare Pt, Se on Bi-covered Pt, and Bi on Se-covered Pt were studied by cyclic voltammetry and coulometry. A steady deposition of Bi₂Se₃ could be attained after negatively stepped adjusting of underpotential deposition (UPD) potentials of Bi and Se on Pt in the first 40 deposition cycles. X-ray diffraction (XRD) analysis indicated that the films were single phase Bi₂Se₃ compound with orthorhombic structure, and the 2:3 stoichiometric ratio of Bi to Se was verified by EDX quantitative analysis. The optical band gap of the as-deposited Bi₂Se₃ film was determined as 0.35 eV by Fourier transform infrared spectroscopy (FTIR), which is consistent with that of bulk Bi₂Se₃ compound.